sounds/looks like a piggy to me:
"The Hartge 'Engine Conversion' is a bespoke tuning module that plugs in ahead of the engine computer on the wiring harness. It has factory-style plugs to connect in-line with the water temp sensor, fuel rail pressure, turbo boost pressure sensors, turbo wastegate valve, intake camshaft sensor, 12V power and ground. The module then sends a modified signal to the engine computer allowing it to increase boost by varying amounts depending on load, engine speed, and other factors."

Pity Hartge don't make this clear on their own website, and to be honest, that's a lot of cash for a piggyback system........??

Am in two minds now

Edit:
I've just looked again at the German site, and now I know what I'm looking for, you can see the N55 kit is a piggyback unit, like the one they do for the diesel engines, whereas the N54 tune isn't and is a reprogrammed ECU unit, which is why I assumed the N55 was as well.